Response of understory plant communities to timber and fire management in a mixed-pine hardwood forest

Background/Question/Methods

Low-intensity fires were historically recurrent in mixed-pine hardwood habitats of North America. In the absence of fire, open savannas and prairies were converted to closed-canopy forests with low understory plant diversity and limited forage for wildlife. The Pushmataha Forest Habitat Research Area was established in the Ouachita Mountains of Oklahoma to test an array of forest management and prescribed fire regimes for large-scale application in restoration of fire-suppressed habitats. This study examined the effectiveness of these restoration efforts 17 and 18 years post-treatment by measuring understory plant community composition and species richness. We predicted that timber harvesting and frequent fires over a long time period are necessary to restore mixed-pine hardwood forests to their historical state (i.e., savannas and prairies). Eight treatments were replicated three times in a completely randomized design and sampled using 0.1 ha nested plots. The treatments were: (1) control (no fire or timber harvesting); (2) four-year interval, low-intensity, late dormant season burns (no timber harvest); (3) harvest Pinus echinata only and annual burns; and (4) five, harvest Pinus echinata and thin hardwoods treatments with no burn, or four-, three-, two-, or one-year burn intervals. 

Results/Conclusions

Regardless of timber harvesting, burned and unburned treatments differed in species composition and richness. Timber management alone had no effect on understory species composition or richness as compared to controls. Species richness was significantly higher in burned than unburned treatments. Shorter fire intervals maximized species richness at all spatial scales. One- and two-year burn intervals prevented the establishment of woody species in the canopy and resulted in prairie habitat. Three-year burn intervals created savanna habitat with patchy and clumped establishment of woody species. In the four-year burn intervals, open woodlands appeared to be forming at a higher density. We conclude that restoration of species richness in fire-suppressed, mixed-pine hardwood forests is possible with prescribed fire alone after extended periods of time. Restoration of structure and function in the short term for species of special concern (such as woodland-grassland obligate songbirds) requires alteration of stand structure through thinning to open the canopy and restore these habitats to their historical state. A three-year burn interval appears to be a threshold, shifting community structure from herbaceous to woody dominated. Therefore, we recommend varying fire frequency across a landscape to create a mosaic of savanna and prairie habitats.